Hydroxyl radicals dominate reoxidation of oxide-derived Cu in electrochemical CO2 reduction

Cuδ+ sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO2 reduction reaction (CO2RR). However, the underlying reason for the dynamically existing Cuδ+ species, although thermodynamically unstable under reductive CO2RR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH•) formed at room temperature in HCO3- solutions. In combination with in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling, we demonstrate a dynamic reduction/reoxidation behavior at the surface of OD-Cu and reveal that the fast oxygen exchange between HCO3- and H2O provides oxygen sources for the formation of OH• radicals. In addition, their continuous generations can cause spontaneous oxidation of Cu electrodes and produce surface CuOx species. Significantly, this work suggests that there is a “seesaw-effect” between the cathodic reduction and the OH•-induced reoxidation, determining the chemical state and content of Cuδ+ species in CO2RR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CO2RR.

Overall, the work is an important and significant contribution to the study of Cu electrocatalyst employed for CO2 reduction. As the authors accurately state in the Introduction, the prior literature has what might appear to be conflicting reports ranging from complete reduction of Cu oxides under CO2R conditions vs. evidence that some oxides, presumably metastable, persist at relatively negative potentials. In this context, authors provide a plausible mechanism for the formation of such oxide layers. Moreover, they posit that there might be a "see-saw" effect in which there is an interplay between the rate of the reduction reactions and the formation of the oxidizing OH radicals.
Technically, the work is excellent, with impressive and thorough characterization. Raman spectroscopy clearly show the re-oxidation ( Figure 1) and I found the use of isotopic labeling to be elegant in supporting the peak assignments ( Figure 4). The scavenging experiments using vitamin C are convincing in implication OH radical in the re-oxidation (dark oxidation experiments are also convincing, inset of Figure 3b) I support publication of this ms. subject to the following minor revisions. Authors must discuss the thermodynamic plausibility of the carbonate/water "interplay" (line 230) that produces the OH radicals, even if they do not have a calculation to support their reasoning. Indeed, as the authors state in the Introduction, "CuOx phases should be removed under the CO2RR operations thereby [causing] the loss of the active Cuδ+ species." Thus an observation that would seem to challenge this thermodynamic view requires some additional comment and, if possible, an explanation. A similar comment applies to the observation of H radicals in the bicarbonate electrolyte solution, page 5.
In the same vein, is the presence of Cu (or a metal surface) required to generate the radicals? That is, the experiments described in lines 108-129 have a Cu electrode presence. Would radicals be made in a similar amount if the metal were not present?
Minor revisions, not technical in nature.
In Figure 2 (and in most of the EPR spectra), the fits shown in color are more prominent than the data, which is shown with a thin line in light grey. The figure should be modified such that the experimental data is more prominent than the fit.
Line 179, replace "corrosives" with "corrosion products." Line 147, replace"OH adduct disappeared, instead of a" with OH adduct disappeared and was replaced by" Reviewer #3 (Remarks to the Author): The major claim of this manuscript is that during CO2RR, HCO3^-and H2O reacts to give OH radicals. These OH radicals are proposed to oxidize the Cu surface during CO2RR. Experimental evidence presented include the use of in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling.
Overall, I do not agree with the claims of this study, as presented in the manuscript. There are major gaps that need to be addressed, before this manuscript can be re-considered. Major: -Authors claimed that HCO3^-and H2O reacts to give OH radicals. And the OH radicals are responsible for oxidizing the Cu surface. But this does not make sense, since in other electrolytes such as KOH (without any HCO3^-present), the Cu surfaces will also quickly oxidize once cathodic potentials are removed.
-Pg 5-Are the authors suggesting that HER occurs at OCP on Cu? How can this be? What is the OCP? Is H2 gas detected? Can the authors use gas chromatography to verify this?
-What is the lifetime and stability of the OH radical? How can it be so long that its signal keeps increasing (Suppl Fig 5)?
-Could a HCO3^-concentration dependent studies be done? I will assume that if the authors are correct, a higher HCO3^-concentration will lead to faster oxidation of the Cu surface.
-What is the mechanism for OH radical formation from HCO3^-+ H2O?
-End of pg 6 and start of Pg 7: The authors do not realize that water can also serve as a source of O to oxidize Cu. It does not need to be O2 gas or OH radical (if these do exist). Minor: -In the introduction of the manuscript, the authors wrote statements such as 'This is due to the partially charged Cuδ+ species that plays a crucial role during CO2RR ', 'Cuδ+ species has been frequently detected during CO2RR.', etc. I think this is an unbalanced introduction. The existence of Cuδ+ species during CO2RR is still controversial. Papers that claim that this species exist often based their claim on results obtained from ex-situ methods. Since Cu oxidizes so easily, the data obtained from ex-situ methods are questionable. I suggest that the authors try to balance out their discussion.
- Fig 1: I suggest that authors show their spectra to at least 2500 cm-1, so that the C-O stretching vibration of CO(ad) at about 2000 cm-1 can be seen.

Dear Reviewers:
We appreciate your careful review and constructive comments which help us to significantly improve the quality of this manuscript (NCOMMS-21-46259). Now we have carefully considered all your comments and revised the manuscript accordingly. We made corresponding corrections point by point as follows (corrections in the revised manuscript are marked in blue color).

Reviewer #1 (Remarks to the Author):
This research suggests that the presence of OH˙ radical in the HCO 3 solution is the key to causing CuO x species during CO 2 RR. However, the presence of OH˙ was detected by using ex situ electron paramagnetic resonance. The result cannot support the claim that OH˙ radical will cause the oxidation of Cu during CO 2 RR. Most importantly, the author did not detect any oxide species under cathodic potential by using in situ Raman spectroscopy ( Figure 1). Moreover, Raman spectroscopy is widely known as surface-sensitive technique. The Raman results obviously contradict the authors" claim that the OH˙ radical affects the chemical state of Cu during CO 2 RR. This research only can conclude that Cu is prone to oxidize under KHCO 3 electrolyte compared to other conditions under OCP due to the presence of OH˙ radical. Based on the aforementioned reasons, this research did not meet the requirement for publishing in Nat. Commun. The followings are more suggestions: Responses 1: • We highly appreciate the suggestions to improve the quality of this paper. We are sorry that we did not clearly describe the key points leading to misleading.
• Regarding electron paramagnetic resonance (EPR) tests, the final goal of this work is to clarify whether the HCO 3 electrolyte can activate OH˙ radicals via the excitation of room temperature. Here two points should be considered: (1), The excitation of OH˙ radicals does not need to apply cathodic potential yet the production of such as H˙ radicals require cathodic potentials. It means that the formation of OH˙ radicals in the bulk electrolyte can not be influenced by the cathodic potentials except for the situation at the electrode interface, where the electrolyte-activated OH˙ could either react with H˙ or get one electron from the electrode to form OH -. Thus, in principle, we just need to test electrolytes with EPR directly, but we cannot exclude the pre-introduced oxidative species, such as O 2 , or pre-existing OH˙ radicals.
(2), So to confirm that the OH˙ radicals were newly generated, we applied cathodic potential at -0.3 V for 20 min. While we added DMPO as a spin trapping agent to increase the lifetime of radicals from ms to tens of mins. Then the applied cathodic potential and the electrochemically generated DMPO-H can gradually eliminate pre-introduced oxidative species, with an indicator of only the DMPO-H signal. Then we stopped the cathodic potential, and after that, we claim that the newly generated DMPO-OH signal was indeed activated in the HCO 3 electrolytes. We added one sentence on pages 5 and 6 in the main text as follows: "Based on this carefully designed test protocol, we claim that the newly generated OH • radicals were indeed activated in the HCO 3 electrolyte." • We have demonstrated the strongly oxidizing OH˙ radicals in the HCO 3 aqueous solutions that lead to the rapid reoxidation of OD-Cu electrodes. However, the argument is whether the reoxidation behavior takes place during CO 2 RR. As suggested, we supplemented in-situ Raman to detect the oxidative species at the surface of the OD-Cu electrode ( Figure R1). The Raman spectra show such as the newly formed Cu-OH vibrations at 710 cm -1 , which could be caused by OH • radical, during CO 2 RR from 0.2 to -0.3 V RHE . The supplementary Raman is shown in Figure S2 and Figure S3 on page S3 and page S4 in the Supporting Information and correspondingly the discussion has been added on pages 3 and 6 in the main text as follows: "In addition, the peaks at 282, 360, 2070-2100 cm -1 are related to the frustrated ρ(Cu−C−O) rotational mode, ν(Cu−CO) stretching mode, and intramolecular C≡O stretching vibration of CO intermediates respectively. The bands at 2820-2950 cm -1 are assigned to the -CH x stretching regions from 0.2 to -0.6 V RHE ( Fig. 1a and   1. In the manuscript, the authors try to eliminate the undesired oxidative species by applying cathodic potential. For instance, in line 101, "To preclude the effect of a trace amount of undesired oxidative species, such as residual O 2 in the electrolyte or at the electrode surface, we implemented a long-time reduction with Cu electrode in CO 2 saturated KHCO 3 solution at -0.3 V RHE for 5 min." and in line 112, "To eliminate the pre-introduction of any oxidative groups, a 20 min of long time reduction on the Cu electrode at -0.3 V RHE was performed in CO 2 saturated 0.5 M KHCO 3 containing 100 mM DMPO.". The authors should unify the conditions for eliminating oxidative species. Most importantly, the authors should prove that they chose the right condition for eliminating any oxidative species in the electrolyte.

Responses 2:
• Thanks very much for your suggestion. To keep the same conditions, 20 min electrochemical reduction was applied. The optical-microscopic images were re-measured, as shown in Figure R2. The same conclusion is achieved. The re-measured result is shown in Figure S7 on page S8 in the Supporting Information and correspondingly the revision has been finished on page 5 in the main text.
• In this study, -0.3 V RHE was selected to eliminate the oxidative species in the solutions. When we did not detect any oxidative species except for H • radical in the electrolyte via the EPR test (Figure 2a), it is the right condition because the newly generated OH • radical can be recorded and confirmed. We agree that a longer reduction time than 20 min or more negative potential than -0.3 V RHE is acceptable, and the conditions do not influence the conclusions. 2. The authors carried out the control experiments in both pure water and K 2 SO 4 to show that HCO 3 is the key for generating OH • radicals. However, I consider that it is much crucial to show that Cu did not oxidize in these conditions. For instance, the authors should carry out the in-situ Raman experiments mentioned in Fig. 1 for the conditions regarding in line 130-136.

Responses 3:
• Thanks very much for this suggestion. It is a good idea to provide in-situ Raman to show the reoxidation processes as shown in Fig. 1. As suggested, we supplemented in-situ Raman in the K 2 SO 4 electrolyte, and no discernable CuO x species was detected at OCP after the reduction of Cu 2 O to metallic Cu at -0.3 V RHE ( Figure R3). It coincides with the EPR result that no OH • radicals were formed in the K 2 SO 4 solution. The supplementary Raman is shown in Figure S6 on page S7 in the Supporting Information and correspondingly the discussion has been added on page 4 and page 7 in the main text as follows: "This rapid reoxidation phenomenon indicates a strongly oxidative species existing in the CO 2 -saturated KHCO 3 electrolyte, in contrast to the non-reoxidation process of OD-Cu in the Ar-saturated 0.25 M K 2 SO 4 electrolyte ( Supplementary Fig. 6)." (page 4) "Meanwhile, the reoxidation of surface OD-Cu in 0.25 M K 2 SO 4 electrolytes was not observed via in-situ Raman ( Supplementary Fig. 6)." (page 7) • Since the reduction of Cu 2 O to metallic Cu at -0.3 V RHE cannot be implemented in pure water owing to the ultrahigh solution resistance, we cannot study the reoxidation process through in-situ Raman. Instead, compared to that in the HCO 3 solution, we have carried out Cu oxidation experiments in the pure water and K 2 SO 4 solutions for 24 h, respectively. We also provided the SEM, EPR, EDS, optical microscopic images, and XPS evidence (  3. Regarding the content mentioned in line164 to line 167, "To further ascertain Cu oxidation associated with electrolyte-induced OH • , the control experiments were carried out in KHCO 3 solution containing VC, pure water, and K 2 SO 4 solution, respectively. No obvious color changes were visually seen for the three cases (insets in Fig. 3  4. According to the authors" Raman results, it is metallic Cu conducting the CO 2 RR reaction. There were no evidence in this research showing that the oxide species can retain during the reaction. As a result, I do not consider that the authors can suggest the presence of Cu δ+ species during CO 2 RR.

Responses 5:
• Thanks very much for this suggestion. As discussed in Responses 1, we proposed that the chemical state of Cu and/or the phase of surface Cu species in CO 2 RR are the results of dynamic equilibrium between the cathodic reduction and the reoxidation caused by strongly oxidative OH • radicals in KHCO 3 electrolytes. We provide evidence of what could be the oxidative species leading to the formation of the Cu δ+ species reported in previous works. As we mentioned in the introduction, "thermodynamically, CuO x phases should be removed under the CO 2 RR conditions thereby the loss of the active Cu δ+ species, but the Cu δ+ species still can be frequently observed". We thus propose that the OH • radical has the chance to get one electron from the Cu catalyst surface.
• At -0.3 V RHE , the cathodic reduction was more dominant than the reoxidation reaction, thus the ultimate chemical state of surface Cu species is mainly metallic Cu. However, it does not mean that the reoxidation reaction does not exist during this process. For example, the Cu-OH vibration still can be observed via in-situ Raman at -0.3 V RHE ; At -0.1 V RHE which is much lower than the reduction potential of Cu 2 O to metallic Cu (0.45 V RHE ), we still clearly observed the Cu-O vibration via in-situ Raman ( Figure R1).
• We chose -0.3 V RHE as the study potential because we found that the reduction rate of Cu 2 O was moderate for in-situ Raman tests. In addition, at this potential, the CO 2 RR has taken place, and this potential can avoid the formation of violent gas bubbles.
5. Since the authors try to understand whether the oxide species of oxide-derived copper can retain during CO 2 RR, the author should carry out the experiments by using "oxide-derived copper catalyst" instead of Cu plate. Reference such as, J. Am. Chem. Soc. 2012, 134, 17, 7231-7234. would make the result more persuasive.

Responses 6:
• Thanks very much for this suggestion. This work used the term "OD-Cu" because we found that the surface of the Cu-mesh electrode had been oxidized to Cu 2 O due to the exposure to air after electrodeposition and to the HCO 3 electrolyte before applying potentials for CO 2 RR. The surface Raman test and the further supplementary HRTEM ( Figure R5) demonstrate the highly oxidized state. The XRD result may lead to a misunderstanding due to the interference of the Cu-mesh matrix.
• In this work, we have noticed the work from the mentioned reference (J. Am. Chem. Soc. 2012, 134, 17, 7231-7234, ref. 4 in the main text). However, we did not use that method to prepare OD-Cu, because that method cannot produce the electrodes with a powerful surface Raman enhancement effect. Instead, we used the electrodeposition method to deposit the Cu micro-nano particles onto the surface of the Cu-mesh substrate. Figure R5. HRTEM of as-prepared OD-Cu electrode.

Reviewer #2 (Remarks to the Author):
S. Mu et al. examine experimentally the oxidation of Cu electrocatalyst in CO 2 -saturated aqueous electrolytes. By using a trapping strategy, they provide evidence of the formation of OH • radicals, to which they attribute the re-oxidation of Cu at the open circuit potential (OCP). Overall, the work is an important and significant contribution to the study of Cu electrocatalyst employed for CO 2 reduction. As the authors accurately state in the Introduction, the prior literature has what might appear to be conflicting reports ranging from complete reduction of Cu oxides under CO 2 RR conditions vs. evidence that some oxides, presumably metastable, persist at relatively negative potentials. In this context, authors provide a plausible mechanism for the formation of such oxide layers. Moreover, they posit that there might be a "see-saw" effect in which there is an interplay between the rate of the reduction reactions and the formation of the oxidizing OH • radicals. Technically, the work is excellent, with impressive and thorough characterization. Raman spectroscopy clearly show the re-oxidation ( Figure 1) and I found the use of isotopic labeling to be elegant in supporting the peak assignments ( Figure 4). The scavenging experiments using vitamin C are convincing in implication OH radical in the re-oxidation (dark oxidation experiments are also convincing, inset of Figure 3b) Responses 1: We truly appreciate your encouraging comments and evaluation of the novelty of our current results.
I support publication of this ms. subject to the following minor revisions. Authors must discuss the thermodynamic plausibility of the carbonate/water "interplay" (line 230) that produces the OH • radicals, even if they do not have a calculation to support their reasoning. Indeed, as the authors state in the Introduction, "CuOx phases should be removed under the CO 2 RR operations thereby [causing] the loss of the active Cu δ+ species." Thus an observation that would seem to challenge this thermodynamic view requires some additional comment and, if possible, an explanation. A similar comment applies to the observation of H • radicals in the bicarbonate electrolyte solution, page 5.

Responses 2:
• Thanks very much for this suggestion. We supplemented more experiments to investigate the excitation source for the generation of OH • radicals in KHCO 3 aqueous solutions. We found that temperature is a key parameter. The room temperature (~25 ℃) is enough for the formation of OH • radicals ( Figure R6). The supplementary EPR is shown in Fig. 3 on page 8 in the main text and correspondingly the discussion has been provided on page 7 in the main text as follows: "In general, the production of radicals requires excitation sources, yet the signal of DMPO-OH adduct has been tracked at ambient conditions. In this case, the most probable excitation source should be temperature. We hypothesize that the room temperature may activate HCO 3 solutions to produce OH • radicals. To confirm it, we implemented the temperature-dependent EPR measurements from 5 to 40 ℃ in Ar-saturated 0.5 M KHCO 3 electrolyte containing 100 mM DMPO (Fig. 3)   • Further, considering that the HCO 3 aqueous solution itself is crucial for the generation of OH • radicals, we supplemented experiments to investigate the relationships between HCO 3 concentration, the intensity of OH • radicals, and reoxidation dynamics of OD-Cu electrodes ( Figure R7). We found that the HCO 3 contributes to the generation of OH • radicals that oxidize the Cu surfaces. At < 0.1 M, there is a linear

the hydrogen evolution reaction (HER) occurs and the generated hydrogen radicals (H • ) can be trapped as a DMPO-H adduct (hyperfine splitting constants, A N = 1.65 mT, A H = 2.25 mT)"
• We further discussed the "reduction hysteresis" of Cu 2 O to metallic Cu. The discussion has been added on page 4 in the main text as follows: "We found that the surface Cu species go through the process: Cu 2 O → CuO x → metallic Cu, with the cathode potential decreasing from OCP to (Supplementary Fig. 2). It is a reverse process when switching from -0.3 V RHE to OCP. Thus, we suggest that the chemical state of Cu and the phase of surface Cu species are the results of dynamic equilibrium between the cathodic reduction and the reoxidation caused by a sort of strongly oxidative species in KHCO 3 electrolytes." In the same vein, is the presence of Cu (or a metal surface) required to generate the radicals? That is, the experiments described in lines 108-129 have a Cu electrode presence. Would radicals be made in a similar amount if the metal were not present? Responses 3: • Thanks very much for this question. The presence of a Cu electrode is not necessary. The electrochemical reduction on the Cu electrode is to remove the pre-existing oxidative substances in the electrolytes. If the metal electrode was not present, a similar amount of OH radicals were produced. For example, the experiments of Supplementary Fig. 9 (here left in Figure R8) and Supplementary Fig. 11 (here right in Figure R8). Minor revisions, not technical in nature.
In Figure 2 (and in most of the EPR spectra), the fits shown in color are more prominent than the data, which is shown with a thin line in light grey. The figure should be modified such that the experimental data is more prominent than the fit.

Responses 4:
Thanks very much for this suggestion. We have modified the EPR spectra and now the experimental data is prominent.
Line 147, replace "OH adduct disappeared, instead of a" with OH adduct disappeared and was replaced by" Responses 6: We have replaced "OH adduct disappeared, instead of a" with "OH adduct disappeared and was replaced by".

Magnetic field (G)
Experimental data DMPO-OH

Reviewer #3 (Remarks to the Author):
The major claim of this manuscript is that during CO 2 RR, HCO 3 and H 2 O reacts to give OH • radicals. These OH • radicals are proposed to oxidize the Cu surface during CO 2 RR. Experimental evidence presented include the use of in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling.
Overall, I do not agree with the claims of this study, as presented in the manuscript. There are major gaps that need to be addressed, before this manuscript can be re-considered.

Responses 1:
We highly appreciate the suggestions to improve the quality of this paper. Major: Authors claimed that HCO 3 and H 2 O reacts to give OH • radicals. And the OH • radicals are responsible for oxidizing the Cu surface. But this does not make sense, since in other electrolytes such as KOH (without any HCO 3 present), the Cu surfaces will also quickly oxidize once cathodic potentials are removed.

Responses 2:
• Thanks very much for this question. We agree with the referee that the Cu surfaces may be quickly oxidized in the other solutions. Nevertheless, in this work, we focus on "why Cu δ+ species can be frequently observed in HCO 3 solutions in previous reports", and "what could be the oxidative species".
• We used the in-situ Raman spectra to study the reoxidation behaviors of OD-Cu in the KHCO 3 solution containing VC and the K 2 SO 4 solution after the reduction of surface Cu 2 O to metallic Cu at -0.3 V RHE ( Figure R9) respectively, where no OH • radicals in both solutions. We found that there are no reoxidation phenomena taking place in both solutions. The supplementary Raman data of the K 2 SO 4 solution is shown in Supplementary Fig. 6 on page S7 in the Supporting Information and correspondingly the discussion has been added on page 4 and page 7 in the main text as follows: "This rapid reoxidation phenomenon indicates a strongly oxidative species existing in the CO 2 -saturated KHCO 3 electrolyte, in contrast to the non-reoxidation process of OD-Cu in the Ar-saturated 0.25 M K 2 SO 4 electrolyte ( Supplementary Fig. 6)." (page 4) "Meanwhile, the reoxidation of surface OD-Cu in 0.25 M K 2 SO 4 electrolytes was not observed via in-situ Raman test (Supplementary Fig. 6)." (page 7) • We carried out more experiments to study the excitation source for the generation of OH • radicals in KHCO 3 aqueous solutions. We found that temperature is a key parameter. The room temperature (~25 ℃) is enough for the formation of OH • radicals ( Figure R10). The supplementary EPR is shown in Fig. 3 on page 8 in the main text and correspondingly the discussion has been provided on page 7 in the main text as follows: "In general, the production of radicals requires excitation sources, yet the signal of DMPO-OH adduct has been tracked at ambient conditions. In this case, the most probable excitation source should be temperature. We hypothesize that the room temperature may activate HCO 3 solutions to produce OH • radicals. To confirm it, we implemented the temperature-dependent EPR measurements from 5 to 40 ℃ in Ar-saturated 0.5 M KHCO 3 electrolyte containing 100 mM DMPO (Fig. 3). When the temperature is as low as 5 ℃, no EPR signals were detected. An obvious EPR signal from DMPO-OH was observed around 20 ℃ and increased with the enhanced temperature. This result indicates that temperature is a key parameter for OH • radical generation in the KHCO 3 solutions." Figure R10. Temperature-dependent EPR spectra in 0.5 M KHCO 3 solutions containing 100 mM DMPO.  "As shown in Fig. 2a, DMPO-H radical adduct generated during the HER did not disappear as soon as we stopped the bias, owing to the increased lifetime, thus they can be measured even if we switched the potential from -0.3 V RHE to OCP." • OCP represents open circuit potential, which has been provided on page 3 and in Fig. 1 on page 4, in the main text. • After stopping the applied bias, no HER takes place, so no H 2 can be detected by gas chromatography.
-What is the lifetime and stability of the OH • radical? How can it be so long that its signal keeps increasing (Suppl Fig 5)? Responses 4: • Thanks very much for this question. The lifetime of OH • radicals is very short (up to several ms), which is far below the time-resolution of EPR. Therefore, we added DMPO to trap OH • radicals via the formation of DMPO-OH adducts, whose lifetime is much longer.
• We agree with the referee that the EPR intensity of DMPO-OH will decrease with increasing time if there are no newly generated OH • radicals. In this case, OH • radicals from HCO 3 solutions are produced all the time under the excitation of room temperature while the concentration of DMPO is enough (100 mM), thus the newly formed OH • radicals can be trapped by DMPO, leading to an increase of DMPO-OH concentration thereby increasing the EPR signal.
-Could a HCO 3 concentration dependent studies be done? I will assume that if the authors are correct, a higher HCO 3 concentration will lead to faster oxidation of the Cu surface.

Responses 4:
• Thanks very much for this question. We supplemented more experiments to investigate the relationships between HCO 3 concentration, the intensity of OH • radicals, and the reoxidation dynamics of OD-Cu electrodes ( Figure R11). We found that the HCO 3 contributes to the generation of OH • radicals that oxidize the Cu surfaces. At < 0.1 M, there is a linear relationship between the intensity of DMPO-OH and HCO 3 concentration. At > 0.1 M, the intensity of DMPO-OH decreases. Thus, HCO 3 should not be the only factor for the generation of OH • radicals since there is no linear relationship between HCO 3 concentration and the intensity of DMPO-OH within the whole HCO 3 concentration range. For example, cation K + might be considered as well when the KHCO 3 concentration is high. Thus, we proposed that the OH • radicals in HCO 3 electrolytes were generated under the excitation of room temperature while the dynamic oxygen exchange between HCO 3 and H 2 O is important. The supplementary data is shown in Fig.   5 on page 11 in the main text and correspondingly the discussion has been made on pages 10 and 13 in the main text as follows: "According to the above analysis, HCO 3 anions play a vital role in determining the OH • generation.
We thus studied the relationships between HCO 3 concentration, the intensity of OH • radicals, and reoxidation dynamics on OD-Cu electrodes. Firstly, we investigated the influence of HCO 3 concentrations on the amount of OH • radicals. As shown in Fig. 5a, at < 0.1 M, the DMPO-OH signal increases with increasing the HCO 3 concentrations, yet further increasing the concentrations cannot produce more OH • radicals. The optimal HCO 3 concentration for the formation of OH • radicals is around 0.1 M (Fig. 5b).

Then, to correlate the OH • radicals with reoxidation behaviors of OD-Cu electrodes, we carried out in-situ Raman to study the reoxidation phenomena in different concentrations of HCO 3 solutions. As
shown in Fig. 5c-  Correspondingly, the normalized intensity of DMPO-OH versus HCO 3 concentration. c-d Real-time Raman spectra of OD-Cu reoxidation at OCP after reduction at -0.3 V RHE in different concentrations of KHCO 3 .
-What is the mechanism for OH • radical formation from HCO 3 -+ H 2 O?
Responses 5: • Thanks very much for this question. We supplemented more experiments to investigate the excitation source for the generation of OH • radicals in KHCO 3 aqueous solutions. We found that temperature is a key parameter. For instance, the low temperature of 5 ℃ cannot cause the generation of OH • radicals, but the room temperature (~25 ℃) is enough for the formation of OH • radicals ( Figure R10). The supplementary EPR is shown in Fig. 3 on page 8 in the main text and correspondingly the discussion has been provided on page 7 in the main text as follows: " • Based on the in-situ Raman, the oxidation of Cu to Cu 2 O can take place within 60 s. It is a fast process. Within 24 h, the thickness of the oxide layer could be around 500 nm. Minor: -In the introduction of the manuscript, the authors wrote statements such as "This is due to the partially charged Cu δ+ species that plays a crucial role during CO 2 RR ", "Cu δ+ species has been frequently detected during CO 2 RR.", etc. I think this is an unbalanced introduction. The existence of Cu δ+ species during CO 2 RR is still controversial. Papers that claim that this species exist often based their claim on results obtained from ex-situ methods. Since Cu oxidizes so easily, the data obtained from ex-situ methods are questionable. I suggest that the authors try to balance out their discussion.

Responses 7:
• Thanks very much for this suggestion. We replaced the previous statements with "This is likely due to the partially charged Cu δ+ species that plays a crucial role".
• We agree with your view that it has not reached a consensus on whether Cu δ+ species can exist at catalyst surfaces or be stable under the cathodic electrolysis in CO 2 RR. We supplemented a discussion about this topic. It has been provided on page 2 in the main text as follows: " To focus on the oxidation of Cu, we did not provide the wide Raman spectra in the previous version. We supplemented the wide Raman spectra and detected the vibrations of C≡O at 2070-2100 cm -1 and the vibrations of CH x at 2820-2950 cm -1 , from 0.2 to -0.6 V RHE ( Figure R12). The supplementary Raman data were shown in Supplementary Fig. 3 (Fig. 1a and b, Supplementary Figs. 2-4) 30,31 , demonstrating the occurrence of CO 2 RR." 2. Why did the authors choose K2SO4 as a controlled measurement? The authors should provide a robust explanation in the manuscript.

Responses 2:
• We thank the reviewer for this kind suggestion. K2SO4 was carefully chosen as a control because: (1) it has moderate solubility (~0.6 M) relative to such as KClO4 (~0.12 M with saturation); (2) SO4 2anion is relatively stable; (3) SO4 2has relatively weaker interaction with Cu relative to such as KCl and KI.
• We added this supplementary information into the main text on page 4: "Here K2SO4 was selected as a control electrolyte because of its moderate solubility relative to KClO4, suitable chemical stability, and relatively weaker interaction with Cu relative to such as KCl and KI." 3. The authors should provide experimental setups of in situ Raman and EPR.

Responses 3:
• We thank the reviewer for this suggestion, and it is helpful for the readers to quickly understand the experimental methods. The supplementary experimental setup images of in-situ Raman have been provided ( Figure S2d on page S4 in Supporting Information).
• EPR instrument equipped with the capillary tube has been provided ( Figure S8b on page S10 in Supporting Information).  4. On page 8 line 190 "To preclude the CO2(aq) effect, the same operation was implemented in Ar-saturated KHCO3, and a darker Cu surface was obtained (inset in Fig. 4 cc). Thus, the Cu oxidation by CO2(aq) was ruled out.", However, the pH varies by purging different gas. Can the authors rule out this effect?

Responses 4: •
We thank the reviewer for this question. This sentence was to introduce that CO2(aq) did not induce the generation of DMPO-OH. We observed that the CO2-purged electrolyte even induces a slightly lower intensity of DMPO-OH (Fig. 4 in the main text) relative to that in the Ar-purged electrolyte.
• We agree that based on the CO2/HCO3equilibrium, theoretically, it is challenging to control the pH and CO2(aq) concentration simultaneously. We found that the pH value of CO2-saturated KHCO3 is around 7.5, while the pH value of Ar-purged KHCO3 is about 8.2. When we refill the Ar-saturated KHCO3 with CO2 for a short duration, similar pH can be retained and similar results can be observed, thus we conclude that the pH effect is not significant here.
• To improve the manuscript, we rewrote the discussion in the main text on page 8.
"To preclude the CO2(aq) effect, the same operation was implemented in Ar-saturated KHCO3, and a darker Cu surface was observed associated with stronger DMPO-OH (inset in Fig. 4c)." 5. On page 10 line 239, "However, HCO3 is not the only factor for the generation of OH radicals since there is no linear relationship between HCO3 concentration and the intensity of DMPOOH.", can the authors provide other potential factors for the generation of OH radicals?

Responses 5:
• Thanks very much for this question. We found that the HCO3contributes to the generation of OH • radicals that oxidize the Cu surfaces. At < 0.1 M, the intensity of DMPO-OH increases with HCO3concentration. At > 0.1 M, the intensity of DMPO-OH decreases.
• Upon increasing the KHCO3 concentration, the K + concentration has been increased as well. Thus, we found that, at the same K + concentration (0.5 M), the relationship between the HCO3concentration and the intensity of DMPO-OH is more pronounced ( Figure R4) following the order 0.2 M < 0.4 M ≤ 0.5 M, and then the intensity of DMPO-OH tends to saturate.
• We supplemented the discussion in the main text on page 10.
"It is worth noting that further increasing the HCO3concentration enhances the K + concentration as well. Thus, at the same K + concentration (0.5 M) with K + compensation by K2SO4, the relationship between the HCO3concentration and the intensity of DMPO-OH is more pronounced following the order 0.2 M < 0.4 M ≤ 0.5 M ( Supplementary Fig. 10), and the intensity of DMPO-OH tends to saturate. The optimal HCO3concentration without K + compensation for the OH • radical formation is around 0.1 M (Fig. 5b)." Figure R4 (Supplementary Fig. 10). EPR spectra of the KHCO3/K2SO4 mixed solutions. The solutions at different HCO3 -/SO4 2mole ratios contain 100 mM DMPO under the same K + concentrations of 0.5 M. Intensity (a.u.)

Magnetic field (G)
Experimental data DMPO-OH